Abstract:

One embodiment of the present invention provides a system actively cancels
vibrations in a computer system. During operation, the system monitors
vibrations in the computer system. Next, the system analyzes the
vibrations to identify one or more harmonics in the vibrations. The
system then actively cancels one or more of the identified harmonics.

Claims:

1. A method for actively canceling vibrations in a computer system, the
comprising:monitoring vibrations in the computer system;analyzing the
vibrations to identify one or more harmonics in the vibrations;
andactively canceling one or more of the identified harmonics.

2. The method of claim 1, wherein actively canceling the identified
harmonics involves using a mechanical transducer to actively cancel the
identified harmonics.

3. The method of claim 2, wherein the mechanical transducer is an
offset-mass motor.

5. The method of claim 2, wherein using the mechanical transducer to
actively cancel an identified harmonic involves adjusting a frequency of
a canceling vibration from the mechanical transducer so that the
frequency of the canceling vibration substantially matches the frequency
of the identified harmonic.

6. The method of claim 2, wherein using the mechanical transducer to
actively cancel an identified harmonic involves adjusting a phase of a
canceling vibration from the mechanical transducer so that the canceling
vibration is substantially opposite in phase to the identified harmonic.

7. The method of claim 2, wherein using the mechanical transducer to
actively cancel an identified harmonic involves adjusting an amplitude of
a canceling vibration from the mechanical transducer so that the
amplitude of the canceling vibration substantially matches the amplitude
of the identified harmonic.

8. The method of claim 1, wherein identifying the one or more harmonics
involves identifying one or more harmonics within a frequency range which
affects hard disk drive performance.

9. The method of claim 1, wherein analyzing the vibrations to identify the
one or more harmonics involves performing a Fast Fourier Transform on the
vibrations.

10. The method of claim 1, wherein the mechanical transducer is coupled to
a hard drive in the computer system so that the mechanical transducer
cancels vibrations at the hard drive.

11. An apparatus that actively cancels vibrations in a computer system,
the comprising:a monitoring mechanism configured to monitor vibrations in
the computer system;an analyzing mechanism configured to analyze the
vibrations to identify one or more harmonics in the vibrations; anda
vibration-cancellation mechanism configured to actively cancel one or
more of the identified harmonics.

12. The apparatus of claim 11, wherein the vibration-cancellation
mechanism includes a mechanical transducer configured to actively cancel
the identified harmonics.

13. The apparatus of claim 12, wherein the mechanical transducer is an
offset-mass motor.

15. The apparatus of claim 11, wherein the vibration-cancellation
mechanism is configured to adjust a frequency of a canceling vibration so
that the frequency of the canceling vibration substantially matches the
frequency of the identified harmonic.

16. The apparatus of claim 12, wherein the vibration-cancellation
mechanism is configured to adjust a phase of a canceling vibration so
that the canceling vibration is substantially opposite in phase to the
identified harmonic.

17. The apparatus of claim 12, wherein the vibration-cancellation
mechanism is configured to adjust an amplitude of a canceling vibration
so that the amplitude of the canceling vibration substantially matches
the amplitude of the identified harmonic.

18. The apparatus of claim 11, wherein the analysis mechanism is
configured to identify one or more harmonics within a frequency range
which affects hard disk drive performance.

19. The apparatus of claim 11, wherein the analysis mechanism is
configured to perform a Fast Fourier Transform on the vibrations.

20. The apparatus of claim 12, wherein the mechanical transducer is
coupled to a hard drive in the computer system so that the mechanical
transducer cancels vibrations at the hard drive.

21. A computer system that actively cancels vibrations, the comprising:a
processor;a memory;a monitoring mechanism configured to monitor
vibrations in the computer system;an analyzing mechanism configured to
analyze the vibrations to identify one or more harmonics in the
vibrations; anda vibration-cancellation mechanism configured to actively
cancel one or more of the identified harmonics.

Description:

BACKGROUND

[0001]1. Field of the Invention

[0002]Embodiments of the present invention relate to techniques for
improving the vibrational health of computer systems. More specifically,
the present invention relates to a method and an apparatus that actively
cancels vibrations that affect the performance of components, such as
hard disk drives (HDDs), within a computer system.

[0003]2. Related Art

[0004]Computer systems such as servers and storage arrays can be adversely
affected by mechanical vibrations that affect internal computer system
components and structures. In particular, when structural resonances are
present in servers or storage arrays at some characteristic frequencies,
it is possible that cooling fans or disk drives operate at rotational
frequencies that are substantially the same as one of the structural
resonant frequencies. When this condition occurs, the rotational motions
of the fans or the disk drives can excite a structural resonance within
the computer system's mechanical structure, thereby causing destructive
amplification of internal vibrations. The amplified internal vibrations
can subsequently lead to degradation of throughput associated with hard
disk drives (HDDs), and can also accelerate other mechanical failure
mechanisms.

[0005]Note that the above-described vibrational problems are becoming more
significant because of the following trends in computer system
manufacturing: (1) cooling fans are becoming increasingly more powerful;
(2) chassis and support structures are becoming weaker because of design
modifications that reduce cost and weight; and (3) internal disk drives,
power supplies, and other system components are becoming more sensitive
to vibration-induced degradation.

[0006]At the same time, HDDs are becoming more sensitive to vibrations
because the storage density for HDDs has increased to the point where a
write head has to align with a track which is less than 20 nanometers
wide. Moreover, the write head floats only 7 nanometers above the disk
surface. These extremely small dimensions make the read and write
performance of the HDDs extremely sensitive to vibrations. Even low
levels of sustained vibrations can significantly deteriorate I/O
performance of the HDDs.

[0007]A "brute force" approach to decouple externally generated vibrations
from the HDDs involves: (1) identifying HDDs which are adversely affected
by the vibrations; (2) identifying the vibration sources; and (3)
inserting rubber or foam dampers, grommets, or stiffeners in available
spaces around identified vibration sources and vibration-sensitive HDDs
in an effort to isolate these components from the rest of the computer
system. However, using elastomeric dampers/stiffeners and grommets is
undesirable because these materials are known to deteriorate with time.
Moreover, the above-described approach can be very costly and inefficient
in practice.

[0008]Hence, what is needed is a method and an apparatus that facilitates
mitigating vibration problems for HDDs without the above-described
problems.

SUMMARY

[0009]One embodiment of the present invention provides a system actively
cancels vibrations in a computer system. During operation, the system
monitors vibrations in the computer system. Next, the system analyzes the
vibrations to identify one or more harmonics in the vibrations. The
system then actively cancels one or more of the identified harmonics.

[0010]In a variation on this embodiment, actively canceling the identified
harmonics involves using a mechanical transducer to actively cancel the
identified harmonics.

[0011]In a further variation on this embodiment, the mechanical transducer
is an offset-mass motor.

[0012]In a further variation on this embodiment, using the mechanical
transducer to actively cancel the identified harmonics involves using
multiple mechanical transducers to cancel multiple identified harmonics.

[0013]In a further variation on this embodiment, using the mechanical
transducer to actively cancel an identified harmonic involves adjusting a
frequency of a canceling vibration from the mechanical transducer so that
the frequency of the canceling vibration substantially matches the
frequency of the identified harmonic.

[0014]In a further variation on this embodiment, using the mechanical
transducer to actively cancel an identified harmonic involves adjusting a
phase of a canceling vibration from the mechanical transducer so that the
canceling vibration is substantially opposite in phase to the identified
harmonic.

[0015]In a further variation on this embodiment, using the mechanical
transducer to actively cancel an identified harmonic involves adjusting
an amplitude of a canceling vibration from the mechanical transducer so
that the amplitude of the canceling vibration substantially matches the
amplitude of the identified harmonic.

[0016]In a variation on this embodiment, identifying the one or more
harmonics involves identifying one or more harmonics within a frequency
range which affects hard disk drive performance.

[0017]In a variation on this embodiment, analyzing the vibrations to
identify the one or more harmonics involves performing a Fast Fourier
Transform on the vibrations.

[0018]In a variation on this embodiment, the mechanical transducer is
coupled to a hard drive in the computer system so that the mechanical
transducer cancels vibrations at the hard drive.

BRIEF DESCRIPTION OF THE FIGURES

[0019]FIG. 1 illustrates a computer system in accordance with an
embodiment of the present invention.

[0020]FIG. 2 illustrates a disk drive with attached accelerometers and
shakers in accordance with an embodiment of the present invention.

[0022]The following description is presented to enable any person skilled
in the art to make and use the invention, and is provided in the context
of a particular application and its requirements. Various modifications
to the disclosed embodiments will be readily apparent to those skilled in
the art, and the general principles defined herein may be applied to
other embodiments and applications without departing from the spirit and
scope of the present invention. Thus, the present invention is not
limited to the embodiments shown, but is to be accorded the widest scope
consistent with the claims.

Computer System

[0023]FIG. 1 illustrates a server 100 that includes multiple fans and
multiple HDDs in accordance with an embodiment of the present invention.
More specifically, server 100 includes a number of cooling fans, i.e.,
fan 102, fan 104, fan 106, and fan 108. These cooling fans are deployed
to increase air circulation around heat-generating components in server
100 and to pump heat out of server 100. Such heat-generating components
can include: the CPU, memory modules, the power supply, and hard disk
drives (HDDs). Hence, fans 102-108 may be placed at different locations
inside server 100. Note that although server 100 is used for the purposes
of illustration, embodiments of the present invention can be applied to
other computer systems, such as desktop computers, workstations, storage
arrays, embedded computer systems, automated manufacturing systems, and
other computer systems which use one or more cooling fans for system
cooling.

[0024]Server 100 also includes an HDD array 110, wherein one or more of
the HDDs in HDD array 110 are sensitive to mechanical vibrations within
server 100. Note that although we show one HDD array in FIG. 1, some
computer systems can include more than one HDD array, while other
computer systems can include one or more HDDs which are not necessarily
configured as an array. Note that mechanical coupling can exist between
each of fans 102-108 and a given HDD in HDD array 110. Furthermore,
mechanical coupling can exist between a pair of HDDs in HDD array 110.

[0025]As cooling fans become increasingly more powerful, they can create a
significant amount of mechanical vibrations in the computer system. In
the following discussion, we use terms "vibration" and "mechanical
vibration" interchangeably. We also use the term "fan speed" to refer to
the rotation frequency of a fan. Note that because fan blades rotate at a
constant rotational speed in terms of rotations per minute (RPM), this
RPM value gives rise to a corresponding frequency component in an
associated vibration spectrum (in Hz). For example, a fan speed of 6000
RPM creates a mechanical vibration containing a frequency component of
(6000 RPM)/(60 sec)=100 Hz. Note that other frequency components can
exist in the vibrational spectrum which are also associated with the fan
operation. These frequency components can include, but are not limited
to, harmonics (e.g., 1×, 2×, 3×, 4×, etc.) of the
primary frequency, and beat frequencies created by slightly different
speeds between fans in the set of fans.

[0026]Referring back to FIG. 1, server 100 includes an accelerometer 112
placed on or integrated with fan 102. Accelerometer 112 can be used to
detect mechanical vibrations produced by fan 102. Specifically,
accelerometer 112 is configured to pick up vibration signals associated
with fan 102, wherein the vibration signals contain information on the
fan speed (in RPM). The accelerometer can then produce an electrical
signal that replicates the vibration signals from the fan.

[0027]Server 100 also includes another accelerometer 114 which is placed
on or is integrated with HDD 116. Accelerometer 114 can be used to detect
mechanical vibrations associated with HDD 116. Specifically,
accelerometer 114 is configured to pick up vibration signals associated
with HDD 116, wherein the vibration signals contain information about the
spindle rotation speed (in RPM) of HDD 116. The accelerometer can then
produce an electrical signal that replicates the vibration signals
associated with the HDD.

[0028]Note that although we show accelerometers being placed on a
particular fan 102 and on a particular HDD 116, accelerometers can
generally be placed at any location in server 100 to monitor vibrations.
In some embodiments of the present invention, other components in server
100 that are to be protected from destructively amplified resonance
vibrations also include accelerometers. For example, other disk drives,
peripheral boards, system board components, tape drives, ASICs, mounting
brackets, and other components in the system may include one or more
accelerometers. Note that some commercial off-the-shelf (COTS) HDDs are
already being manufactured with one or two internal accelerometers which
can be used to measure the vibration experienced by the HDDs. Also note
that the present invention is applicable to a computer system equipped
with any number of fans and any number of HDDs. Hence, the present
invention is not limited to the particular configuration illustrated in
FIG. 1.

[0029]The outputs from accelerometers 112 and 114 are coupled to a
telemetry device 118, which is capable of gathering electrical signals
produced by the accelerometers and generating time-series signals 120. In
one embodiment of the present invention, telemetry device 118 is part of
a Continuous System Telemetry Harness (CSTH), which provides real-time
outputs for the instrumentation signals. Note that these instrumentation
signals can include other signals associated with physical performance
parameters measured through sensors within the computer system. For
example, the physical parameters can include distributed temperatures
within the computer system, relative humidity, cumulative or differential
vibrations within the computer system, fan speed, acoustic signals,
current noise, voltage noise, time-domain reflectometry (TDR) readings,
and miscellaneous environmental variables.

[0030]These instrumentation signals can also include signals associated
with internal performance parameters maintained by software within the
computer system. For example, these internal performance parameters can
include system throughput, transaction latencies, queue lengths, load on
the central processing unit, load on the memory, load on the cache, I/O
traffic, bus saturation metrics, FIFO overflow statistics, and various
operational profiles gathered through "virtual sensors" located within
the operating system.

[0031]Telemetry device 118 directs time-series signals 120 to a local or a
remote location that contains an analyzer 122. Analyzer 122 is configured
to identify harmonics in the vibration signals received from
accelerometers 112 and 114. More specifically, in one embodiment of the
present invention, analyzer 122 is configured to identify the three most
significant harmonics in the frequency range known to affect HDD
performance (nominally from 300 to 600 Hz in the preferred embodiment).
This analysis operation can involve, for example, performing a Fast
Fourier Transform (FFT) on the vibration signals from accelerometers 112
and 114 to convert the vibration signals into the frequency domain. A
number peaks of the highest amplitude in the frequency domain can then be
identified as the most-significant harmonics.

[0033]One embodiment of the present invention uses very low cost vibration
sources to cancel the principal offending harmonics and provide
mechanical stability assurance throughout the life of the server. More
specifically, server 100 includes vibration sources 126 and 128, which
are transducers that accept an input signal and output mechanical
vibrations. In some embodiments of the present invention, vibration
sources 126 and 128 are Commercial-Off-The-Shelf (COTS) vibrators, such
as the vibrators used in cell phones. In some embodiments, the frequency
of the mechanical vibrations can be varied according to the magnitude of
an input voltage.

[0034]Possible configurations for the accelerometers and vibration sources
are described in more detail below with reference to FIG. 2.

Disk Drive with Attached Accelerometers and Shakers

[0035]FIG. 2 illustrates a disk drive with attached accelerometers and
shakers in accordance with an embodiment of the present invention. In the
embodiment illustrated in FIG. 2, the housing of a hard disk drive (HDD)
202 within a computer system is coupled to a vibration transferring plate
204. Vibration-transferring plate 204 is itself coupled to three
vibration sources (which are implemented as shakers 208) and two
vibration sensors (which are implemented as translational accelerometers
206).

[0036]In one embodiment of the present invention, the three shakers 208
comprise three tiny offset-mass motors which are rigidly affixed to the
housing structure holding one or more hard disk drives (HDD). Note that
three vibration sources are used in one embodiment of the present
invention because it has been empirically verified that canceling the
three highest harmonics in the sensitive frequency range for HDDs
generally results in satisfactory throughput performance from HDDs.

[0037]Also note that the cost of the tiny vibrational stimulus generator
motors shown in the FIGS. 1 and 2 is quite modest and the devices are
known to be easily manufacturable in very large quantities. For example,
the vibrator can be a standard COTS vibrator deployed in over a billion
cell phones; the only modification for our new application is a
variable-voltage control algorithm that matches the frequency of each of
N vibrators to the N highest harmonics in the vibrational spectrum that
is most deleterious to HDD throughput.

[0038]Voltage to the offset-mass motors is controlled so that the three
motors run at the same frequency, but 180 degrees out of phase with
respect to the three corresponding highest harmonics in the internal
vibrational profile for the server. The phase of the motors can be
actively controlled to be 180-degrees out of phase through a number of
well-known techniques. For example, the phase can be adjusted by
temporarily increasing or decreasing the voltage applied to the
offset-mass motors. Moreover, this phase-adjustment can be actively
controlled to cancel vibrations through a standard feedback-control
technique which varies the phase of an offset-mass motor and then
measures the resulting cancellation. Note that machine-learning technique
can be used during this feedback-control process to learn how an
adjustment in phase affects the resulting vibration cancellation.

[0039]Moreover, the amplitude of the vibrations from these COTS vibrators
can be matched to the amplitude of the harmonics by providing offset-mass
motors with different masses, and then associating a specific harmonic
with a specific mass that matches the magnitude of the specific harmonic.
For example, the amplitude of vibrations from standard tiny cell-phone
vibrators has been empirically shown to be more than sufficient for
active stabilization of HDD housing structures even in high-end servers
as large as a refrigerator.

Vibration Cancellation Process

[0040]FIG. 3 presents a flowchart illustrating the process of active
vibration cancellation in a computer system. First, the system monitors
vibrations in the computer system, for example through one or more
translational accelerometers (step 302). Next, the system analyzes the
vibrations to identify one or more harmonics in a frequency-domain
profile for vibrations (step 304). As mentioned previously, this analysis
process can involve performing an FFT to identify the harmonics. The
system then actively cancels one or more of the identified harmonics
using active transducers, such as offset-mass motors (step 306).

[0041]The foregoing descriptions of embodiments of the present invention
have been presented only for purposes of illustration and description.
They are not intended to be exhaustive or to limit the present invention
to the forms disclosed. Accordingly, many modifications and variations
will be apparent to practitioners skilled in the art. Additionally, the
above disclosure is not intended to limit the present invention. The
scope of the present invention is defined by the appended claims.